As well known to those skilled in the art, transfer case assemblies for a four wheel drive vehicle are divided into a manual type in which driving scheme conversion is effected through manipulation of a driver and an electronic type in which driving mode conversion is effected by an electronic signal. In the manual type transfer case assembly, driving scheme conversion of the four wheel drive vehicle is effected as the driver directly manipulates a shift lever disposed adjacent to the driver's seat.
The manual type transfer case assembly is encountered with a problem in that, since the driver should directly manipulate the shift lever, the driver can feel fatigued, it is bothersome to manipulate the shift lever, and an accident is likely to occur while manipulating the shift lever.
In these considerations, in these days, the electronic type transfer case assembly has been widely used throughout the world in that, since gearshift is automatically effected, manipulation can be conducted in a convenient manner, fatigue of the driver can be reduced, and it is possible to prevent an accident from occurring.
A typical example of the conventional electronically controlled transfer case assembly is shown in FIG. 1.
A gearshift device 100 of the electronically controlled transfer case assembly largely comprises a camshaft 102 which is rotated by a motor 101 and is defined with a groove 103, a first fork 104 which functions to convert a wheel driving scheme between a four wheel driving scheme and a two wheel driving scheme, a second fork 105 which is manually operated by a separate lever (not shown) to convert a four wheel driving mode between a four wheel high speed driving mode and a four wheel low speed driving mode, and a gearshift rod 106 having one end which is coupled to the first fork 104 and the other end which is inserted and guided in the groove 103.
In the gearshift device 100 constructed as mentioned above, if the camshaft 102 is rotated as power is applied to the motor 101, the gearshift rod 106 inserted in the groove 103 is moved along the groove 103 to be changed in its position relative to the camshaft 102. As a result of this, the first fork 104 to which the gearshift rod 106 is coupled is moved to force a sleeve (not shown) to move, by which conversion between the four wheel driving scheme and the two wheel driving scheme is effected.
Consequently, as the motor 101 is rotated in forward or backward directions, gearshift is implemented into the four wheel driving scheme or the two wheel driving scheme.
In the meanwhile, with the wheel driving scheme converted into the four wheel driving scheme by the first fork 104, when it is necessary to convert the four wheel driving mode between the four wheel high speed driving mode and the four wheel low speed driving mode, the position of the second fork 105 is moved by operating the separate lever, by which the four wheel driving mode can be converted from the four wheel high speed driving mode into the four wheel low speed driving mode, and vice versa.
However, the conventional gearshift device 100 suffers from defects in that, since the groove 103 should be defined with a high precision in the camshaft 102, it is difficult to properly define the groove 103. Further, due to the fact that first gearshift means for effecting conversion of the wheel driving scheme between the four wheel driving scheme and the two wheel driving scheme and second gearshift means for effecting conversion of the four wheel driving mode between the four wheel high speed driving mode and the four wheel low speed driving mode should be separately provided, as the number of parts is increased, a manufacturing cost is increased, and inconvenience is caused due to the need of operating the separate lever.
Also, referring to U.S. Pat. No. 6,155,126 describing another example of a gearshift device for the electronically controlled transfer case assembly, a shift fork body is slidably coupled to a shift rail. When the shift rail is rotated, as the shift fork body is moved forward and backward in an axial direction of the shift rail, a shift collar coupled to a web member is moved forward and backward to effect desired gearshift.
Nevertheless, the conventional construction described just above has a drawback in that, since a cam for actually moving the shift fork body has a configuration of a helical thread, its machining procedure is complicated. Moreover, the helical thread has a predetermined slope, it is impossible to implement gearshift in a diversity of ways.
Referring to U.S. Pat. No. 6,173,624 describing still another example of a gearshift device for the electronically controlled transfer case assembly, an inclined surface of a predetermined slope is formed on a cam, and a cam follower is attached to an outer surface of a rail, so that gearshift is effected as a camshaft is rotated.
Nonetheless, the conventional construction described just above is not free from shortcomings in that, since the inclined surface of the predetermined slope has a fixed contour, it is difficult to effect gearshift in a diversity of ways. In particular, because the cam follower, that is, a projection is formed on the outer surface of the rail, a machining procedure is complicated, and inconvenience results from the need of changing a configuration of the entire rail.
In addition, because a fork is returned using one spring, it is difficult to promptly and precisely return the fork into its original position.